1. Field of the Invention
The present invention relates to a driving process for a liquid crystal display, and in particular to a driving process for an active matrix type liquid crystal display which is suitable for motion picture display.
2. Background Art
In recent years, liquid crystal displays (hereafter abbreviated as LCD) have increased in size and definition, and the range of images displayed is also widening, from the handling of mainly still images such as in the liquid crystal displays used with personal computers and word processors and the like, to incorporate the handling of motion pictures such as in the liquid crystal displays used as televisions and the like. An LCD is thinner than a TV equipped with a CRT (cathode ray tube), and can be installed without occupying much space, and consequently it is expected that LCDs will become widely used in average households.
FIG. 20 shows a sample construction of a conventional active matrix type LCD. The LCD comprises a first and a second glass substrate, and a liquid crystal display panel section 100 for displaying images. A number n (where n is a natural number) of scanning lines 101 and a number m (where m is also a natural number) of signal lines 102 are disposed in a grid like arrangement on top of the first glass substrate, and a TFT (thin film transistor) 103 which functions as a non linear element (switching element) is provided in the vicinity of each point of intersection between the scanning lines 101 and the signal lines 102.
A gate electrode of the TFT 103 is connected to the scanning line 101, a source electrode is connected to the signal line 102, and a drain electrode is connected to a pixel electrode 104. The aforementioned second glass substrate is then arranged in a position facing the first glass substrate, and a common electrode 105 is then formed on one surface of the glass substrate with a transference electrode of ITO or the like. Then, a liquid crystal is used to fill the space between the common electrode 105 and the pixel electrode 104 formed on the top of the first glass substrate.
The scanning lines 101 and the signal lines 102 are connected to a scanning line driving circuit 106 and a signal line driving circuit 107 respectively. The scanning line driving circuit 106 sequentially drives a large electric potential to the n scanning lines 101, and switches the TFT 103 connected to each scanning line 101 to an ON state. With the scanning line driving circuit 106 in the scanning state, the signal line driving circuit 107 outputs a gradation voltage corresponding with the image data to one of the m signal lines, and the gradation voltage is written to the pixel electrode 104 via the TFT 103 in an ON state, and the potential difference between the common electrode 105 which is set at a uniform potential, and the gradation voltage written to the pixel electrode 104 is used to control the amount of light transmission and consequently the display. The liquid crystal display panel section 100 is driven in this manner.
FIG. 21 is a diagram showing waveforms of signals output from the scanning line driving circuit 106 and the signal line driving circuit 107 of a conventional liquid crystal display to the scanning lines 101 and the signal lines 102 respectively. In FIG. 21, the symbols VG1 to VGn represent scanning signal waveforms applied to each of the scanning lines 101. As shown in the figure, the scanning signals VG1 to VGn apply a high electric potential to only one scanning line 101 at any one time, and the signals are output sequentially to the n scanning lines 101. Furthermore, the symbol VD represents a signal output to a single signal line 102, and the symbol Vcom represents a signal waveform applied to the common electrode 105. In the example shown in FIG. 21, the signal VD is a signal in which the signal strength varies in accordance with each piece of image data, whereas the signal Vcom is of a uniform value and does not vary over time.
Furthermore, in such a liquid crystal display, in order to prevent the deterioration of the liquid crystal, so-called AC driving is used, and generally the device is controlled so that a DC component voltage is never applied to the liquid crystal for a long period of time. One example of an AC drive method involves making the voltage applied to the common electrode 105 uniform, and applying alternate positive polarity and negative polarity signal voltages to the pixel electrode 104.
If motion picture display is conducted on this type of LCD, then problems of image quality deterioration, such as the residual image phenomenon, will arise. The cause of this problem is that because the response speed of the liquid crystal material is slow, when a gradation variation occurs, the liquid crystal is unable to track the gradation variation within a single field period and produces a cumulative response using several field periods. Consequently, considerable research is being conducted into various high speed response liquid crystal materials as a way of overcoming this problem.
However, the aforementioned problems such as the residual image phenomenon are not caused solely by the response speed of the liquid crystal, and have also been reported by institutions such as the NHK Broadcasting Technology Research Laboratory as being caused by the display process (for example, refer to the 1999 Conference of the Electronic Information Communication Society, SC-8-1, pp.207-208). As follows is a description of this problem of the display process, with a comparison of a CRT driving process and an LCD driving process.
FIGS. 22A and 22B are diagrams showing comparative results for the time response of display light of a pixel in a CRT and an LCD, where FIG. 22A shows the time response for a CRT, and FIG. 22B shows the time response for an LCD. As shown in FIG. 22A, the CRT is a so-called in-pulse type display device where light is generated for only several milliseconds from the time the electron beam strikes the fluorescent substance of the tube surface, whereas the LCD shown in FIG. 22B is a so-called hold type display device where the display light is retained for one field period from the time the writing of data to the pixel has finished until the next write occurs.
When motion pictures are displayed on a CRT and an LCD with the above characteristics, the displays shown in FIGS. 23A and 23B results. FIGS. 23A and 23B are diagrams showing a sample image display in the case where motion pictures are displayed on a CRT and an LCD, where FIG. 23A represents a sample CRT display and FIG. 23B represents a sample LCD display. FIG. 23A and FIG. 23B represent the case of a circular display object moving in a direction x shown in the figures. In such a case, then as shown in FIG. 23A, in the in-pulse type display device CRT, the display object is displayed momentarily at positions corresponding with the time, whereas in a hold type display device LCD the image of the previous field remains until immediately before a new write is performed.
When a person views the motion pictures displayed in the manner shown in FIGS. 23A and 23B, then the motion pictures are perceived in the manner shown in FIGS. 24A and 24B. FIGS. 24A and 24B are diagrams describing the image perceived by a person when a motion picture is displayed on a CRT and an LCD, where FIG. 24A represents the case of a CRT, and FIG. 24B represents the case of an LCD. As shown in FIG. 24A when a motion picture is displayed on an in-pulse type display device CRT, there is no perception at any time of a displayed image overlapping the previous image. However, when a motion picture is displayed on a hold type display device LCD, then due to effects such as the time integral effect of human sight, the currently displayed image is perceived to overlap with the previously displayed image, producing a motion blur problem.
Several improvements have been proposed for overcoming the aforementioned problems which arise when motion pictures are displayed on an LCD. One such improvement is a method where by scanning the scanning lines at a multiple speed, a new image can be written during the period of each field, and motion blur consequently reduced (multiple scan method). However this multiple scan method also suffers from problems in that the frequency becomes very high, and the circuit size increases due to the necessity of creating a new image to be inserted between fields.
Another improvement is a method in which a shutter is provided in the light path of the display and the hold time is shortened (shutter method). In this method, then for example in the case of a transmission type LCD, the back light is flashed and motion blur prevented by blocking the light for a proportion of a single field period.
Furthermore, another process has also been proposed (for example, Japanese Unexamined Patent Application, First Publication No. Hei 10-83169) in which a black image which functions as a shutter is inserted between each set of image data.
FIGS. 25A to 25D are diagrams describing a process of preventing motion blur by inserting a black image between each set of image data. As shown in FIG. 25A, the basis of this process comprises applying a predetermined voltage to the liquid crystal to generate a black display during a horizontal blanking period, and therein prevent motion blur. In other words, following the display of an image for one field, the entire screen is switched to a black display, before the image of the next field is displayed. However, when display is carried out according to this process, the display time differs in a direction perpendicular to the liquid crystal display panel 100, and so as shown in the sample panel display in FIG. 25C, the problem arises of a difference in brightness developing depending on the position on the liquid crystal display panel 100.
Processes for suppressing this difference in brightness have been proposed in Japanese Unexamined Patent Application, First Publication No. Hei 9-127917, Japanese Unexamined Patent Application, First Publication No. Hei 10-62811 and Japanese Unexamined Patent Application, First Publication No. Hei 11-30789, among others. FIG. 26 is a diagram showing the construction of a liquid crystal display for resolving the problem which develops in the process shown in FIG. 25A. The construction shown is that proposed in the aforementioned Japanese Unexamined Patent Application, First Publication No. Hei 9-127917. Those structural elements which are identical with those of the conventional liquid crystal display shown in FIG. 20 are labeled with the same symbols.
FIG. 26 represents the conventional circuit construction shown in FIG. 20 to which has been added a black display write circuit comprising a black signal supply section 120, a black signal supply line 121, a black signal supply scanning line 122, a black signal supply TFT 123 and a scanning line driving circuit 124 for driving the black signal supply scanning line 122. The gate electrode of the black signal supply TFT 123 is connected to the black signal supply scanning line 122, the source electrode of the black signal supply TFT 123 is connected to the black signal supply line 121, and the drain electrode is connected to the drain electrode of the TFT 103 and the pixel electrode 104.
In a liquid crystal display of the above construction, within one field, a voltage corresponding with a black display is applied to the pixel electrode 104, and then a voltage corresponding with the image data is applied to the pixel electrode 104. By using this type of driving process, each scanning line is reset in the same manner as the panel display example shown in FIG. 25B. In other words, following the display of one screen image, rather than performing a reset by switching the entire screen to a black display, by performing the reset in units of scanning lines, the occurrence of a difference in brightness resulting from insertion of a black screen, such as that shown in the panel display example shown in FIG. 25D, is prevented.
In this manner, using the circuit shown in FIG. 26, motion blur can be reduced, and any difference in brightness across the screen can be prevented, but with such a construction, in addition to the conventional liquid crystal display shown in FIG. 20, the black signal supply section 120, the black signal supply line 121, the black signal supply scanning line 122, the black signal supply TFT 123 and the scanning line driving circuit 124 are necessary, and so the circuit construction increases in size which invites problems such as a reduction in the panel numerical aperture.
The object of the present invention is to provide a driving process for a liquid crystal display which prevents motion blur without resulting in an increase in circuit size or a reduction in panel numerical aperture.
In order to achieve the object, the present invention is a driving process for a liquid crystal display in which a plurality of scanning lines and a plurality of signal lines are disposed in a grid like arrangement, and display of an image corresponding with image data is conducted by selecting any one of the scanning lines at one time, and altering the state of a liquid crystal via the signal line, wherein a first scanning period and a second scanning period are set within a time frame shorter than the time necessary for scanning any one of the aforementioned scanning lines, and an image corresponding with the aforementioned image data is displayed via the aforementioned signal line during the first scanning period, and a monochromatic image is displayed via the aforementioned signal line during the second scanning period.
According to the present invention described above, a driving process for a liquid crystal display is provided in which a plurality of scanning lines and a plurality of signal lines are disposed in a grid like arrangement, and display of an image corresponding with image data is performed by selecting any one of the scanning lines and the signal lines at one time, and altering the state of a liquid crystal, wherein a first scanning period and a second scanning period are set within a time frame shorter than the time necessary for scanning any one of the aforementioned scanning lines, and an image corresponding with the aforementioned image data is displayed via the aforementioned signal line during the first scanning period, and a monochromatic image is displayed via the aforementioned signal line during the second scanning period, and as a result the present invention is able to prevent the appearance of motion blur without any increase in circuit size or any reduction in panel numerical aperture.
In the present invention, in relation to the same scanning line, the first scanning period and the second scanning period may be set with a time separation therebetween, and an image corresponding with the aforementioned image data may be displayed during the first scanning period of a scanning line, and a monochromatic image may be displayed during the second scanning period of a scanning line which is separated by a predetermined number of scanning lines from the scanning line which displayed the aforementioned image.
Furthermore in the present invention, the aforementioned monochromatic image may be displayed across a predetermined number of consequitive scanning lines.
Furthermore in the present invention, signals relating to an image corresponding with the aforementioned image data and the monochromatic image may be output alternately to the aforementioned signal line, and a signal relating to an image corresponding with the aforementioned image data may be output with an inversion in polarity at every aforementioned first scanning period, and a signal relating to the aforementioned monochromatic image may be output with an inversion in polarity at every aforementioned second scanning period.
Furthermore in the present invention, the aforementioned monochromatic image may be a black image.
Furthermore in the present invention, the aforementioned liquid crystal may be constructed so that the display state thereof is white when no voltage is applied and gradually alters to a black display state in accordance with an applied voltage, and moreover the liquid crystal may be positioned between a pixel electrode and a common electrode, and the voltage applied between the pixel electrode and the common electrode when displaying the black image during the aforementioned second scanning period may be greater than the voltage applied between the pixel electrode and the common electrode when producing a black display during the aforementioned first scanning period.
Furthermore in the present invention, the voltage applied between the aforementioned pixel electrode and the aforementioned common electrode may be made variable by holding the voltage applied to the common electrode at a uniform level, and increasing the voltage applied to the pixel electrode via the aforementioned signal line.
Furthermore in the present invention, the voltage applied between the aforementioned pixel electrode and the aforementioned common electrode may be made variable by applying a voltage to the pixel electrode via the aforementioned signal line, and varying the voltage applied to the common electrode.
Furthermore in the present invention, the aforementioned scanning lines may be connected to a plurality of scanning line driving circuits, and the scanning lines may be scanned in sequence by two scanning line driving circuits selected from amongst the plurality of scanning line driving circuits, and during the aforementioned first scanning period, the scanning of one of the aforementioned two selected scanning line driving circuits may be stopped, and during the aforementioned second scanning period, the scanning of the other of the two selected scanning line driving circuits may be stopped.